The invention relates to porous membrane filters, in particular to micro- and ultraporous membrane filters which have fluid-impermeable areas on one side due to a change of the members on that side into a film-like state.
Membrane filters made of a wide variety of thermoplastic materials or polymers are known in the prior art. Thermoplastic materials can include cellulose acetates, polyamides, polyvinylidene fluoride or polysulfone, which have the most varying pore sizes and pore size distributions and can be symmetrical or asymmetrical, as well as hydrophilic or hydrophobic. Typical examples of such membrane filters are described in EP 96 306.
Such membrane filters are frequently brittle, tolerate only very slight mechanical stress, and are sensitive to tearing or tear propagation. As a result, various problems arise in handling, for example, when cutting, stamping, or in the assembly of filter elements such as filter cartridges, plate or pillow modules. When membrane filters are positioned preferably in a pleated manner in filter cartridge housings, the positioning is accomplished by embedding in cold or heat curing multi-component resins or by embedding in melts of synthetic thermoplastics and then solidifyinq the sealing material. In this process, changes can occur in the physical properties of the membrane filter within and immediately outside the fixing area. For example, previously hydrophobic membrane filters can become hydrophilic, such as in the case of the surfactant effect of adsorbed low-molecular resin components. Also, previously hydrophilic membrane filters can become hydrophobic, such as when coating the hydrophilic surface of the membrane filters with absorbed low-molecular resin components and/or when surfactants are desorbed under the influence of temperature, wherein a change in the crystalline portion of partially crystalline polymers and other morphological changes due to the influence of temperature can also play a role. These problems that particularly arise during the manufacture of filter cartridges are described in detail in the aforementioned EP 96 306. This application discusses the problem that arises in the manufacture of cartridges when hydrophilic membrane filters, such as nylon membrane filters, are embedded into polypropylene melts. The problem is that the membrane filters are made hydrophobic beyond the embedding area into an edge zone, whereby this edge zone that is non-wettable with water contains a great number of pores that are a bypass for air in the bubble area test or pressure holding test (integrity test) of the filter cartridge. The air permeability makes the testability of the filter cartridge impossible.
To solve in particular the latter problem, costly processes exist in the prior art that can reduce significantly the porosity of membrane filters at specific desired areas and/or avoid a transition from hydrophilic to hydrophobic, or vice versa.
Thus, for example, US 3,407,252 describes a membrane filter in which the edge area is covered by applying a strip made of a hot-sealable, cross-linkable epoxide resin (hot-setting adhesive).
Furthermore, it has been known to cover the edge of a polyvinylidene fluoride membrane that is to be pleated and built into a filtration module with a polypropylene film which is pressed by mechanical means on the membrane. In the process, the polypropylene film and/or the membrane filter are in a swollen state due to the solvent, and the solvent has to then be removed by means of evaporation.
The aforementioned EP 96 306 describes the sealing of the edge of hydrophilic membrane filters, such as nylon filters, by means of a heat-sealable polyester film, which on the one side has a solvent-free polyethylene surface coating as a hot-setting adhesive.
Furthermore, a process for reducing the edge porosity of membrane filters is proposed in this application. In the process, a casting solution with different components, such as nylon solutions, is poured to obtain a filter that, in addition to the porous filter areas, has edge strips of less porosity.
EP 96 306 also describes the reduction of porosity at the edge strips of the membrane filters in which two superimposed filter webs are mechanically molded whereby the micro- or ultraporous filter matrix collapses over the entire filter thickness.
In addition to the "heat-seal" and the mechanical process, EP 00 36 315 also mentions a process in which the sensitive area of the porous filters is treated by coating with glue adhesive.
Despite the fact that the above described, known processes are very costly (with the exception of the process in which a fluid-impermeable film is applied to one side of the membrane filter), it is not possible to produce on one side of a filter a fluid-impermeable structure and still retain on the opposite side a porous structure. Such a structure is desired, for example, when in the process of embedding the membrane filter into a melt, said melt is to penetrate the porous structure, thereby creating an ideal anchorage possibility.
Of course, the known process of pressing a fluid-impermeable material on a membrane filter offers the possibility of making, on the one hand, the membrane filter fluid-impermeable at one desired area and, on the other hand, making available on the opposite side a porous structure. However, this process, like most of the other above-described processes, necessitates that an outside material be introduced into the membrane filter or applied on the membrane filter, which, in many cases is undesired.
Therefore, the technical problem on which the invention is based is to provide porous membrane filters that are made fluid-impermeable at desired areas in a simple manner without introducing or applying outside materials, but still maintain at those areas opposite the fluid-impermeable areas the filter's porous membrane structure.